1. Field of the Invention
The present invention relates to a filter device, and more particularly, a filter device constituted of an acoustic wave element such as a surface acoustic wave (SAW) element having a resonance characteristic in the VHF/UHF band.
2. Description of the Related Art
The filter device is widely used in the communication field. In particular, the filter device using a surface acoustic wave (SAW) resonator, or a SAW filter (hereinafter generically termed SAW element), is largely used in mobile communication such as mobile telephone, because of being able to be compact in size with a low loss.
In recent years, mobile telephone becomes to have high functionality, and TV broadcast has become able to be watched by a mobile phone, with an analog TV tuner or a tuner for one-segment broadcast mounted thereon. However, when viewed from the TV tuner side, a mobile telephone signal, particularly a transmission signal of mobile telephone becomes a cause of trouble such as crosstalk, while when viewed from the mobile telephone side, a TV signal becomes a cause of trouble such as crosstalk.
To cite cases in European countries, TV signals exist in the VHF band (10-200 MHz) and the UHF band (470-750 MHz), while the transmission signals of mobile telephone exist in the 800 MHz band (880-915 MHz) and the 1.8 GHz band (1,710-1,785 MHz). Therefore, on the TV tuner side, there is required a low pass filter to pass the TV UHF band or lower, and to attenuate the 800 MHz band or higher. In contrast, on the mobile telephone side, there is required a high pass filter to pass the 800 MHz band or higher, and to attenuate the TV UHF band or lower.
Moreover, because 750 MHz on the high frequency side of the TV signals and 880 MHz on the low frequency side of the mobile telephone signals are very close to each other, it is necessary to realize a filter having a steep cut-off characteristic.
Further, in the circuit configuration of communication equipment, because elements having balanced terminals such as a frequency converter and an amplifier are generally connected before and after the filter device, the filter device is also required to have balanced input and output.
Conventionally, as a method for realizing a high pass filter or a low pass filter, a method of using a ladder type structure is known (for example, the official gazette of the Japanese Unexamined Patent Publication No. Hei-5-7125, which is hereafter referred to as Patent document 1).
FIG. 1 is a diagram illustrating an equivalent circuit when realizing a high pass filter with the ladder structure, described in the above Patent document 1. The structure is a cascade connection of unit filters, each configured of a series resonant circuit constituted of an inductance L and a capacitance C, and an impedance element Ie. Also, FIG. 2 is a diagram illustrating an equivalent circuit when realizing a low pass filter with a ladder structure described in Patent document 1. The structure is a cascade connection of unit filters, each configured of a parallel circuit of an inductance L and an impedance element Ie.
As can be understood from the equivalent circuits shown in FIGS. 1, 2, the structures of the high pass filter and the low pass filter described in the above Patent document 1 have unbalanced input and output. Further, because of the structure of the unit filters in cascade connection, there is a demerit of an increased number of cascade stages, causing an increased number of elements, to realize a steep cut-off characteristic.